In visual basic, Hashtable is useful to store a collection of key/value pairs of different data types and those are organized based on the hash code of the key.
Generally, the hashtable object will contain buckets to store elements of the collection. Here, the bucket is a virtual subgroup of elements within the hashtable and each bucket is associated with a hash code, which is generated based on the key of the element.
In visual basic, hashtable is same as Dictionary object but the only difference is the Dictionary object is useful to store a key-value pair of same data type elements.
When we compared with Dictionary object, the hashtable will provide lower performance because the hashtable elements are of object type so the boxing and unboxing process will occur when we store or retrieve values from the hashtable.
In visual basic, the hashtable is a non-generic type of collection so we can store key/value pair elements of different data types and it is provided by System.Collections namespace.
As discussed, the collection is a class so to define a hashtable, we just need to declare an instance of the hashtable class before we perform any operations like add, delete, etc. like as shown below.
Dim htbl As Hashtable = New Hashtable()
If you observe the above hashtable declaration, we created a new hashtable (hshtbl) with an instance of hashtable class without specifying any size.
The following are some of the commonly used properties of hashtable in a visual basic programming language.
| Property | Description |
|---|---|
| Count | It is used to get the number of key/value pair elements in the hashtable. |
| IsFixedSize | It is used to get a value to indicate that the hashtable has fixed size or not. |
| IsReadOnly | It is used to get a value to indicate that the hashtable is read-only or not. |
| Item | It is used to get or set the value associated with the specified key. |
| IsSynchronized | It is used to get a value to indicate that access to hashtable is synchronized (thread-safe) or not. |
| Keys | It is used to get the collection of keys in the hashtable. |
| Values | It is used to get the collection of values in the hashtable. |
The following are some of the commonly used methods of hashtable to perform operations like add, delete, etc. on elements of hashtable in a visual basic programming language.
| Method | Description |
|---|---|
| Add | It is used to add an element with a specified key and value in the hashtable. |
| Clear | It will remove all the elements from the hashtable. |
| Clone | It will create a shallow copy of hashtable. |
| Contains | It is used to determine whether the hashtable contains a specific key or not. |
| ContainsKey | It is used to determine whether the hashtable contains a specific key or not. |
| ContainsValue | It is used to determine whether the hashtable contains a specific value or not. |
| Remove | It is used to remove an element with the specified key from the hashtable. |
| GetHash | It is used to get the hash code for the specified key. |
In visual basic, while adding elements to hashtable we need to make sure that there will not be any duplicate keys because hashtable will allow us to store duplicate values but keys must be unique to identify the values in the hashtable.
In hashtable, we can add key/value pair elements of different data types. Following is the example of adding elements to hashtable in visual basic.
Module Module1
Sub Main(ByVal args As String())
Dim htbl As Hashtable = New Hashtable()
htbl.Add("msg", "Welcome")
htbl.Add("site", "Tutlane")
htbl.Add(1, 20.5)
htbl.Add(2, Nothing)
' Another way to add elements. If key not exist, then that key adds a new key/value pair.
htbl(3) = "Tutorials"
' Add method will throws an exception if key already exists in hash table
Try
htbl.Add(2, 100)
Catch
Console.WriteLine("An element with Key = '2' already exists.")
End Try
Console.WriteLine("*********HashTable Elements********")
' It will return elements as KeyValuePair objects
For Each item As DictionaryEntry In htbl
Console.WriteLine("Key = {0}, Value = {1}", item.Key, item.Value)
Next
Console.ReadLine()
End Sub
End Module
If you observe the above example, we created new hashtable (htbl) and adding different data type elements (key/value) to hashtable (htbl) in different ways. As discussed, the Add method will throw an exception in case, if we try to add a key (2) which is already existing so to handle that exception we used try-catch block.
When we execute the above visual basic program, we will get the result as shown below.
If you observe the above result, we got an exception when we tried to add a key (2) which is already existing and added a key (3) which is not existing in the hashtable.
In visual basic, we can also assign key / value pairs to hashtable at the time of initialization like as shown below.
Dim htbl As Hashtable = New Hashtable() From {
{"msg", "Welcome"},
{"site", "Tutlane"},
{1, 20.5},
{2, Nothing}
}
In visual basic, we have different ways to access hashtable elements i.e. either by using keys or by iterating through a hashtable using For Each loop.
Following is the example of accessing hashtable elements in different ways.
Module Module1
Sub Main(ByVal args As String())
Dim htbl As Hashtable = New Hashtable()
htbl.Add("msg", "Welcome")
htbl.Add("site", "Tutlane")
htbl.Add(1, 20.5F)
htbl.Add(2, 10)
Dim msg As String = CStr(htbl("msg"))
Dim num As Single = CSng(htbl(1))
Console.WriteLine("*********Access Elements with Keys********")
Console.WriteLine("Value at Key 'msg': " & msg)
Console.WriteLine("Value at Key '1': " & num)
Console.WriteLine("*********Access Elements with Foreach Loop********")
For Each item As DictionaryEntry In htbl
Console.WriteLine("Key = {0}, Value = {1}", item.Key, item.Value)
Next
Console.WriteLine("*********HashTable Keys********")
For Each item In htbl.Keys
Console.WriteLine("Key = {0}", item)
Next
Console.WriteLine("*********HashTable Values********")
For Each item In htbl.Values
Console.WriteLine("Value = {0}", item)
Next
Console.ReadLine()
End Sub
End Module
If you observe the above example, we are accessing hashtable elements in different ways by using keys, For Each loop and with available properties (Keys, Values) based on our requirements.
When we execute the above visual basic program, we will get the result as shown below.
In visual basic, we can delete elements from hashtable by using the Remove() method. Following is the example of deleting the elements from hashtable in a visual basic programming language.
Module Module1
Sub Main(ByVal args As String())
Dim htbl As Hashtable = New Hashtable()
htbl.Add("msg", "Welcome")
htbl.Add("site", "Tutlane")
htbl.Add(1, 20.5F)
htbl.Add(2, 10)
htbl.Add(3, 100)
htbl.Remove(1)
htbl.Remove("msg")
Console.WriteLine("*********HashTable Elements********")
For Each item As DictionaryEntry In htbl
Console.WriteLine("Key = {0}, Value = {1}", item.Key, item.Value)
Next
Console.ReadLine()
End Sub
End Module
If you observe the above example, we used a Remove() method to delete the particular value of the element from hashtable using keys.
When we execute the above visual basic program, we will get the result as shown below.
In case, if you want to remove all the elements from a hashtable, use Clear() method.
By using Contains(), ContainsKey() and ContainsValue() methods, we can check whether the specified element exists in hashtable or not. In case, if it exists these methods will return True otherwise False.
Following is the example of using Contains(), ContainsKey() and ContainsValue() methods to check for an item that exists in a hashtable or not in visual basic.
Module Module1
Sub Main(ByVal args As String())
Dim htbl As Hashtable = New Hashtable()
htbl.Add("msg", "Welcome")
htbl.Add("site", "Tutlane")
htbl.Add(1, 20.5F)
htbl.Add(2, 10)
htbl.Add(3, 100)
Console.WriteLine("Contains Key 4: {0}", htbl.Contains(4))
Console.WriteLine("Contains Key 2: {0}", htbl.ContainsKey(2))
Console.WriteLine("Contains Value 'Tutlane': {0}", htbl.ContainsValue("Tutlane"))
Console.ReadLine()
End Sub
End Module
If you observe the above example, we used Contains(), ContainsKey() and ContainsValue() methods to check for particular keys and values that exists in a hashtable (htbl) or not.
When we execute the above visual basic program, we will get the result as shown below.
The following are the important points which need to remember about hashtable in visual basic.